Hide the once-move-out privilege for stack fns behind '-Z once-fns'

src/test/compile-fail/once-cant-call-twice-on-stack.rs

@@ -11,6 +11,7 @@
 // Testing guarantees provided by once functions.
 // This program would segfault if it were legal.
 
+// compile-flags:-Z once-fns
 extern mod extra;
 use extra::arc;
 use std::util;

src/test/run-pass/once-move-out-on-stack.rs

@@ -10,6 +10,7 @@
 
 // Testing guarantees provided by once functions.
 
+// compile-flags:-Z once-fns
 extern mod extra;
 use extra::arc;
 use std::util;